This invention relates to internal combustion engines and, in particular, to methods and apparatus for reducing exhaust emissions.
It is well known in the art to use exhaust gas recirculation (EGR) as a means of controlling the emissions of nitrous oxides (NOx) from internal combustion engines. In a typical EGR system, a portion of the exhaust gases (typically from 5 to 15%) is reintroduced into the induction system along with the fresh charge of air and fuel. The exhaust gas, which is essentially inert, displaces the amount of combustible mixture in a gasoline (Otto cycle) engine. In a diesel engine, the exhaust gas replaces some of the excess oxygen in the pre-combustion mixture. Because NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion temperatures caused by the reduction in combustible mixture or excess oxygen reduces the amount of NOx the combustion produces.
In 2002, United States environmental protection agency implemented regulations that required exhaust gas recirculation coolers to be implemented in passenger vehicles and light trucks equipped with diesel engines as a means of further reducing the NOx emissions from these vehicles. Such exhaust gas recirculation coolers are typically of the gas-to-liquid heat exchanger variety and are most often of a shell-and-tube heat exchanger design in which the exhaust gas passes through a plurality of tubes encased in a shell through which the engine coolant circulates. U.S. Pat. No. 8,079,409 and U.S. Pat. No. 7,213,639 are typical of such exhaust gas recirculation cooler designs
Difficulties associated with exhaust gas recirculation coolers in diesel engines include the fact that reducing the combustion temperature increases the amount of soot formed by the combustion process. This soot tends to deposit in the tubes of the exhaust gas recirculation cooler where it acts as an insulating layer that reduces the thermal efficiency of the exhaust gas recirculation cooler. Additionally, if the engine coolant runs low, the heat exchanger may be starved of coolant and may experience a so-called “thermal event” in which the cooler tubes, heated nearly to the temperature of the exhaust gas, thermally expand to a degree that exceeds the structural integrity of the heat exchanger.
Various methods have been suggested to improve the longevity of exhaust gas recirculation coolers, including use of expansion joints, forming the tubes in the shape of elongated bellows and/or manufacturing the exhaust gas recirculation cooler as a series of short modules, each of which has a relatively small overall thermal growth. For example, U.S. Pat. No. 6,460,520 issued to Challis, suggests construction of an EGR cooler in which the shell portion includes a plurality of 90 degree bends formed as corrugated bellows. According to Challis, the bellows sections have increased compliance over a straight-walled shell and, therefore, the bellows provide for better accommodation of thermal expansion or other movements. U.S. Pat. No. 7,213,639 issued to Danielsson et al. suggests an EGR cooler in which the flow of the exhaust gas enters through a central row of tubes and exits through a peripheral row of tubes. According to Danielsson, the reversing flow reduces the risk of local hot spots due to stagnation of coolant flow. German Patent DE 10 2005 058314 A1 (Daimler Chrysler AG) discloses an EGR cooler in which three tubes are formed into tube bundles that are twisted into helixes formed about a common helical axis. The tubes, however, are all wound with the same direction of twist (i.e. all right-hand or all left-hand twist) and are wound about an imaginary rod having a non-zero diameter. Because the tube bundles all have the same direction of twist, it is not possible to position the tube bundles any closer together than an equivalent group of cylinders having the same outside diameter as the tube bundles. Winding the tubes about a central rod having a non-zero diameter, leads to additional spacing inefficiency.